zulip_ops: Disable Ubuntu's built-in update-motd.d files.

We can't really do this in the zulip manifests (since it's sorta a
sysadmin policy decision), but these scripts can cause significant
load when Nagios logs into a server (because many of them take 50ms or
more of work to run).  So we just get rid of them.

puppet/zulip_ops/manifests/base.pp

@@ -65,6 +65,15 @@ class zulip_ops::base {
     mode       => '0600',
   }
 
+  # Clear /etc/update-motd.d, to fix load problems with Nagios
+  # caused by Ubuntu's default MOTD tools for things like "checking
+  # for the next release" being super slow.
+  file { '/etc/update-motd.d':
+    ensure     => directory,
+    recurse    => true,
+    purge      => true,
+  }
+
   file { '/etc/pam.d/common-session':
     ensure     => file,
     require    => Package['openssh-server'],